Relief image printing plates are widely used for printing on a variety of substrates, including paper, corrugated stock, film, foil, and laminates. Relief plates typically include a support layer and one or more layers of cured photopolymer.
In the manufacture of relief printing plates from photocurable materials, a layer of a liquid or solid material undergoes polymerization, cross-linking, or some other curing reaction upon exposure to actinic radiation, usually ultraviolet light. Typically, the actinic radiation first passes through a photographic negative and then through the photocurable material to selectively cure or harden the material in a pattern corresponding to the image borne by the negative. The negative is typically an image-bearing transparency consisting of substantially opaque and substantially transparent areas. Photocuring takes place in the exposed areas, i.e., those areas of the photocurable layer corresponding positionally to the substantially transparent areas of the negative, and little or no photocuring takes place in the unexposed areas. The photocurable layer can also be exposed from the side opposite the negative. Generally, this "back exposure" is not done in an imagewise manner, and used to form a hardened base for the raised printing indicia formed by the front exposure. The exposed layer is then developed by removal of the unexposed, unhardened portions with an air knife, developer solvent, or other means to form a relief image.
In producing relief images from half-tone negatives, it is often necessary to produce relatively small diameter raised printing indicia in what are otherwise recessed areas of the relief. These areas print the lighter or "highlight" areas of the image, and the raised indicia in these areas are accordingly referred to as "highlight dots". The size and density of highlight dots control shading or tone in the light image areas. Half-tone relief images also generally contain relatively shallow, small diameter depressions in what are otherwise overall raised areas of the relief. These areas print the darker or "shadow" areas of the image and the depressions are generally referred to as "shadow reverses". The size and density of shadow reverses control the tone in the darker image areas.
The production of satisfactory highlight dots and shadow reverses in relief images by photoexposure techniques can present unique problems. Due to the small size of the dots in the half-tone negative used to produce highlight dots, it is generally desired to provide a high-image exposure dose in order to assure both formation of the dots and adequate depthwise curing. Such curing generally is needed to anchor the dot to the image base. Shadow reverses typically form beneath relatively small opaque areas of the negative in an otherwise predominantly transparent area. A high exposure dose can result in exposure of at least a portion of the photocurable material underneath the opaque areas of the photonegative. This translates into a shadow reverse with sloped sides. The result is a distorted printed image.
A high image exposure dose also can result in overexposure in areas beyond dot areas of the half-tone negative. This results in increased dot image area, i.e., a relief dot that is larger than the corresponding transparent area of the negative. In addition, the overexposure results in a higher, broader shoulder profile on the dot. This leads to the printing of smudges and larger-than-intended dots, particularly where there is any over-impression during printing.
Although a lower radiation dose can be used to both lessen the detrimental effects to shadow reverses, as described above, and improve dot shoulder geometry, the lower dose often results in unsatisfactory formation and anchoring of the highlight dots. In addition, it has been observed that the highlight dots generally have a greater tendency to move or otherwise deform during the photoexposure step when lower intensity radiation is used. Such dot movement or deformation is believed to be due, at least in part, to shrinkage of the photocurable material as curing occurs. Dot movement is highly undesirable, resulting in positional displacement of the dot and formation of streaks or tails behind the dot. These streaks or tails becomes part of the relief image and adversely affect the quality of images printed with the plate.
For many photocurable systems, there is no exposure dose which satisfactorily provides both highlight dots and shadow reverses, and the operator is thus forced to make undesired compromises in exposure dose and image quality. For other systems, there can be an exposure dose which provides generally acceptable highlight dots and shadow reverses, but this often occurs in a very narrow "window" of doses, thus attaching a very high degree of criticality to the exposure. The narrow window also means that there is a high risk of error in the exposure step, particularly where there is variability in the intensity of the actinic radiation or in the photoresponse of the photocurable material from one lot to another.
Although the foregoing focuses on the problems with image fidelity and resolution encountered in preparing half-tone relief images, it will be appreciated that similar problems can occur in the formation of other types of high resolution relief images, such as line images. Since the formation of relief images by photoexposure involves an image having not only length and breadth dimensions, but also a substantial and significant depth dimension, unique demands are placed on the photosensitive system and the photoexposure method which are not encountered in those processes and systems used to form only two dimensional images, such as in conventional photographic or photocopying systems.
Previously, there have been attempts to enhance the images formed on relief image printing plates. U.S. Pat. No. 5,147,761, issued Sep. 15, 1992, to Wessells et al. ("the Wessells Patent"), herein incorporated by reference in its entirety, discloses a method for preparing relief image printing plates. The method as described by the Wessells Patent involves positioning a louver, or collimator, having a plurality of open cells having a reflective surface over an image-bearing photographic negative which, in turn, is positioned over a photocurable material. The function of the louver is to reflect and redirect the incident rays of radiant energy towards a more parallel path to the image-bearing photographic negative and the photocurable material. As shown in FIG. 1, however, the reflective surface of the louver still directs the radiant energy, A, at an angle obtuse to the image-bearing photographic negative. The result is a relief image that is distorted from its true size relative to the image on the photographic negative.
Consequently, there remains a need in the art for improved methods of forming relief images by photoexposure.